High-temperature internal friction in polycrystalline Zircaloy-4

The high-temperature internal friction spectrum of polycrystalline Zircaloy-4 is investigated in detail, for a wide range of frequencies. Two internal friction maxima are observed. The lower-temperature peak is interpreted in terms of a relaxation mechanism produced by the sliding of particle-free grain boundaries. The higher-temperature peak is attributed to the sliding of boundaries blocked by small precipitates. Values for the activation enthalpy and the preexponential factor for diffusion along grain boundaries are given, and the viscosity coefficient associated with grain-boundary sliding is determined as a function of temperature.     

Atomic-scale interface structure of a Cr-coated Zircaloy-4 material

Highly adherent, thin Cr coatings on Zr-based nuclear fuel claddings can be potentially used for the development of accident-tolerant fuels in light water reactors. To guarantee the successful implementation of Cr-coated Zr alloys as cladding tubes in nuclear power plants, the adhesive strength of the Cr coatings must be assessed. The interface between Cr and Zr was characterized via high-resolution transmission electron microscopy. We observed the formation of nanometer-thick Zr(Fe, Cr)₂ poly-type, structured Laves phases at the interfacial region that display both C14 and C15 lattice symmetries. Although the crystallinity was preserved throughout the interfacial region, different atomic configurations were observed for all the interfaces studied. In most cases, coherent or semicoherent crystallographic relationships were observed, ensuring the adhesive strength of the coating.     

Fracture surface energies of high explosives PETN and RDX

Journal of Materials Science -     

Meso-scale analysis of the creep behavior of hydrogenated Zircaloy-4

During dry storage, creep is the most likely degradation mechanism for spent Zircaloy fuel cladding. The fuel cladding integrity during dry storage depends on the amount of oxidation, irradiation hardening and hydrogen-uptake during in-reactor operation. In this paper, the effect of hydrogen on the creep behavior of Zircaloy-4 cladding material was investigated at different temperatures. Depending on temperature, hydrogen can be found in the sample in solid solution and/or hydride. To capture this phenomenon, a numerical mesoscale model of the hydrogenated material has been built using the Finite Element (FE) Method. The numerical setup explicitly describes the hydrides as an inclusion in a hydrogenated Zircaloy-4 matrix. The matrix creep behavior follows a combined Norton-Bailey and Norton creep rules whereas the hydrides are considered to be elastic material. The creep law was defined in FE Code ABAQUS using the user subroutine CREEP. The comparison of predicted creep behavior obtained from numerical modeling showed good agreement with the results reported in literature. The predicted creep behavior shows a significant effect of hydrides morphology. Particularly, our model is able to seize the competition between the creep strain rate enhancement induced by hydrogen in solid solution and its reduction due to precipitated hydrogen.     

Low cycle deformation behaviour of Zircaloy-4 at elevated temperatures

The LC deformation behaviour of Zry-4 at 400°C and 600°C was examined by means of tension/compression experiments conducted in load and in strain control respectively. The main results were compared to those obtained at comparable conditions on the stainless steel type AISI 304. For both the materials the influence of the stress ratio R = σ_min/σ_max (where within one test σ_max > 0 was kept constant) upon the lifetime T_f at low and high homologeous temperature T_h was examined. Whereas at the lower T_h for R < 0 the lifetime decreased with decreasing R, the opposite was true at the higher T_h. The explanation of the influence of R upon t_f suggests that at high temperatures the fatigue damage rate Å_f drops below the rate for creep damage Å_c Two cases are considered. If the above damage mechanisms are sequentially independent the resulting damage rate Å ≈? Å_c and hence Å_c is the failure (rate) determining mechanism. In the case that the mechanisms are sequentially dependent then Å ≈? Å_f. TEM investigations conducted on Zry-4 cycled at 600° C have shown that the typical dislocation pattern revealed is a band structure consisting of dense dislocation walls separating denuded zones. The habit and the crystallographic characteristics of the band structure resemble the structure associated with PSBs observed in fee metals. The comparison of the values of the saturation stress τ_s and the wall spacing d for different fee and hep metals shows that there is a proportionality between τ_s and 1/d which is independent of stress and temperature.     

Microstructure and electrochemical properties of Zircaloy-4 under Fe ion irradiation

To investigate the microstructure and electrochemical properties of Zircaloy-4 induced by Fe ion irradiation with the energy of 150 keV at liquid nitrogen temperature, transmission electron microscope analysis (TEM) was employed to analyze the surface layer of the samples irradiated at a dose ranging from 1×10¹³ to 1×10¹⁶ ions/cm², and potentiodynamic polarization measurements were used to evaluate the corrosion resistance of Zircaloy-4 in a 1 N H₂SO₄ solution at room temperature. TEM analyses show that Fe ion irradiation lead to a structural change and amorphous phase formation on the surface of the samples. Moreover, it is indicated from the corrosion tests that with an increase of the irradiation dose, the passive current density increases at first and then decreases rapidly, while the natural corrosion potential goes down at first and then up rapidly. The critical point, where the corrosion properties are transformed from a damaging stage to protecting stage, is at the damage level of 3.19 dpa. Finally, the mechanism for the change of corrosion resistance of the Zircaloy-4 samples is discussed.     

Development of substructure in Zircaloy-4 during LCF at 873 K

Transmission electron microscopy techniques were used to study the substructure developed in Zircaloy-4 during strain controlled Low Cycle Fatigue (LCF) test at 873 K. A complete microstructural analysis on samples cycled with a total strain range of 0.01 and total strain rate 2×10^–3s^–1 was made at a different number of cycles. This study revealed that the dislocations arranged themselves into a band structure which remains essentially constant since the first cycle up to fracture. This substructure stability agrees with the stress response observed of this alloy at 873 K. A mechanism to account for the observed structure is proposed.     

Diffusion bonding of Ti coated Zircaloy-4 and 316-L stainless steel

Diffusion bonding of Zircaloy-4 and Type 316-L stainless steel was carried out by coating the joining surfaces with Ti to minimize the interlayer effect. Bonding heat treatments were carried out in vacuum at 1000 °C for 4 h and 1050 °C for 1 h. The microstructure of the diffusion zone was investigated by scanning electron microscopy and the phases in the diffusion zone were analyzed by energy dispersive spectroscopy. It is observed that Ti coating at the interface produced a dendritic structure in the diffusion zone formed in the Zircaloy-4. The concentration of the dendrites increases with an increase in bonding temperature.     

Pulsed laser spot welding of intersection points for Zircaloy-4 spacer grid assembly

A pulsed laser spot welding technology has been developed for joining intersection points of 0.457 mm-thick Zircaloy-4 straps. Weld beads size, mechanical properties and microstructure characterization of the weld beads were investigated. The results indicate that peak power of the pulsed laser has significant influence on the penetration depth of weld beads. Pulse width and number of shots should be taken into consideration mainly to control the weld width. The average value of ultimate tensile loads of the intersection points are continuously increasing as the penetration depth and weld width of weld beads increase. Fracture was located at the fusion line and the fracture surface could be characterized as a mixture of ductile and cleavage feature. Column grains and equiaxed grains were observed in fusion zone and heat affected zone, respectively. The fusion zone consists of a mixture of α-Zr and β-Zr phases among which lamellar Widmanstatten structure α-Zr + Zr₃Fe was distributed. The Zr(Fe, Cr)₂ second phase particles were precipitated inter and intragranular of α-Zr grains.     

Labyrinth structures in Zircaloy-4 as a consequence of strain ageing effects on fatigue

Dislocation structures in Zircaloy-4 samples fatigued at 623, 713 and 823 K were studied by transmission electron microscopy techniques. The samples, cycled with a total strain rate 2×10^–3s^–1, show a linear cyclic hardening rate which is maximum at temperatures where stronger strain ageing effects were reported. A two-wall structure similar to the labyrinth structure observed in fcc metals was found. This structure is attributed to the enhanced dislocation activity produced by strain ageing phenomena.     

Hydrogen in Zircaloy-4: effects of the neutron irradiation on the hydride formation

X-ray diffraction patterns of neutron irradiated Zircaloy-4 samples were obtained at the Brazilian Synchrotron Light Laboratory (LNLS) to study the effects of the fast neutron fluxes and post-irradiation thermal treatments on the zirconium hydride evolution. The high intensity and resolution of the synchrotron beam allowed to detect the (111)_δ and (220)_δ diffraction peaks of the ZrH_1.5 + x (0 ≤ x≤ 0.16) δ-phase in unirradiated Zircaloy-4 samples having a hydrogen concentration as low as 0.2 at.% (20 wppm). Then, irradiated samples, that were taken from the Zircaly-4 core components of the argentine HWPR Atucha I Nuclear Power Plant (CNA-1) were studied with that radiation to detect the diffraction peaks of ZrH_1.5 + x δ-phase particles precipitated in samples having hydrogen isotope concentration lower than 2 at.% (220 wppm). A significant increment of the (111)_δ and (220)_δ peak areas were observed at room temperature after post-irradiation thermal treatments at 600 °C during 4 h. These results indicate that after the annealing a hydrogen concentration between 35 to 70 wppm, which were apparently absent in the irradiated samples has precipitated at room temperature as zirconium hydrides. That amount was estimated from a (220)_δ peak area versus the bulk H concentration regression line, made with unirradiated samples. These results get support to a hydrogen trapping hypothesis proposed in previous works. In addition, it has been shown that the zirconium hydride that precipitate in the irradiated samples have the equilibrium δ-ZrH_1.5 + x cubic crystalline structure.

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Strain rate dependence of iodine-induced stress corrosion cracking of Zircaloy-4 under internal pressurization tests

Pressurization tests were run on unirradiated Zircaloy-4 tubing in the pressure range of 400 to 550 MPa and temperature, 330 to 400 ° C. The effects of the mechanical factor on the susceptibility of Zircaloy to iodine-induced stress corrosion cracking (ISCC) were studied in terms of both time-to-failure and failure strain. The time-to-failure was related to thenth power of stress, and the failure strain was a parabolic function of the strain rate in the limited range of 10^–7 to 10^–4 sec^–1. The ISCC susceptibility was determined by the strain rate rather than the stress, and decreased with increasing test temperature. The results suggest that the film rupture step should be involved in the Zircaloy ISCC and that gas adsorption process is an important step in the overall ISCC.     

An anisotropic Gurson type model to represent the ductile rupture of hydrided Zircaloy-4 sheets

The aim of this work is to model the ductile fracture of Zircaloy-4 sheets containing various amount of embrittling hydride precipitates. The proposed model is based on the Gurson–Tvergaard–Needleman model which is extended to take into account plastic anisotropy and viscoplasticity. The mechanical behavior is identified by conducting tensile tests and the damage nucleation rate (hydride cracking) is measured using quantitative metallography. The model is then used in a Finite Element software to represent crack propagation in Center Crack Panel specimens. Results are strongly mesh size dependent. The mesh size has to be identified by comparison with experimental results. Finally the model is validated by simulating crack initiation and growth in moderately complex structures (sheets containing holes).     

Partial transient liquid phase diffusion bonding of Zircaloy-4 to stabilized austenitic stainless steel 321

An innovative method was applied for bonding Zircaloy-4 to stabilized austenitic stainless steel 321 using an active titanium interlayer. Specimens were joined by a partial transient liquid phase diffusion bonding method in a vacuum furnace at different temperatures under 1 MPa dynamic pressure of contact. The influence of different bonding temperatures on the microstructure, microindentation hardness, joint strength and interlayer thickness has been studied. The diffusion of Fe, Cr, Ni and Zr has been investigated by scanning electron microscopy and energy dispersive spectroscopy elemental analyses. Results showed that control of the heating and cooling rate and 20 min soaking at 1223 K produces a perfect joint. However, solid-state diffusion of the melting point depressant elements into the joint metal causes the solid/liquid interface to advance until the joint is solidified. The tensile strength of all the bonded specimens was found around 480–670 MPa. Energy dispersive spectroscopy studies indicated that the melting occurred along the interface of the bonded specimens as a result of the transfer of atoms between the interlayer and the matrix during bonding. This technique provides a reliable method of bonding zirconium alloy to stainless steel.     

LC4铝合金断裂韧度的研究

通过对LC4铝合金裂纹扩展方向和断裂方式的研究,探讨了LC4铝合金拉伸性能、断裂韧性和微观组织的关系。研究结果表明,LC4铝合金的断裂是由第二相颗粒决定的塑性断裂,第二相颗粒平均间距是断裂结构参数,建立了LC4铝合金拉伸性能、断裂韧性和第二相颗粒平均间距的定量关系。